Electric motors commonly include a stationary component called a stator and a rotating component called a rotor. The rotor rotates within (or around) the stator when the motor is energized with a driving waveform. When the driving waveform is removed from the motor, the rotor may coast to a standstill over time due to the inertia of the rotor and anything coupled to the rotor.
In many motor applications, it is desirable to stop rotation of the rotor as soon as the driving waveform is removed from the motor. For example, in washing machine applications, it is desirable to stop rotation of the washing machine motor after a high speed spin cycle so that the washing machine may be unloaded or switched to a slower speed wash or rinse cycle.
Accordingly, various techniques have been developed for braking electric motors. One such technique uses brake pads, pulleys, and/or other friction braking systems. Unfortunately, friction brakes add cost to a motor and are therefore not desirable for low cost applications such as washing machines. Friction brakes also eventually wear out with use and require repair or replacement.
Thus, many motor applications employ electric braking systems rather than friction brakes. One type of electric braking system employs regenerative braking technology. Although such technology is effective and energy efficient, it is far too complicated and expensive for lower cost applications such as washing machine motors. Another type of electric braking system is DC injection braking in which a direct current (DC) voltage is applied to a motor's stator windings to brake the rotor. The DC voltage creates a stationary magnetic field which applies a static torque to the rotor. This slows and eventually halts rotation of the rotor. As long as the DC voltage remains on the stator windings, the rotor is held in position and resists rotation. DC injection braking is relatively simple, cost-effective, and maintenance free and is therefore a popular choice of braking for many motor applications; however, it has not been used effectively in some applications as described below.
It is also often desirable to determine when a motor's rotor has stopped rotating so the rotor can be driven in the opposite direction, at a different speed, etc. This can be accomplished with a motor shaft sensor such as a Hall effect sensor, but such sensors increase the cost and complexity of motors and are therefore not desirable for many lower cost applications such as washing machine motors.
Thus, sensorless techniques for determining motor speed have been developed. One type of sensorless speed detection employs various algorithms for estimating when a rotor stops based on measured electrical parameters. However, the measured electrical parameters, and thus the results of the algorithms, are less accurate when the motor is being braked with the above-described DC injection braking techniques. Thus, DC injection braking techniques generally require a motor shaft sensor.
Another sensorless technique to ensure a motor has stopped rotating is to simply have a time delay that must lapse after power is removed from the motor. A similar technique uses DC injection braking to slow the motor as described above and continues the DC injection until a time delay has lapsed. The first of these methods unfortunately wastes time between motor cycles because the time delay must account for the maximum possible coast time of a motor, and the second of these methods wastes energy and time because the DC injection braking must be maintained longer to account for the maximum possible breaking time even though the motor may in fact stop sooner.
The above section provides background information related to the present disclosure which is not necessarily prior art.